blk-mq: provide dummy blk_mq_map_queue_type() helper

Doesn't do anything right now, but it's needed as a prep patch
to get the interfaces right.

While in there, correct the blk_mq_map_queue() CPU type to an unsigned
int.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>